Portfolio A expects a 10% return with 15% volatility. Portfolio B expects an 8% return with 10% volatility. Assuming a 2% risk-free rate, which one do you prefer and why? If you choose B, how can you beat A's return?

组合A预期收益10%，波动率15%。组合B预期收益8%，波动率10%。假设无风险利率2%，你选哪个，为什么？如果你选了B，你怎么做才能击败A的收益？